The invention relates to a thermal head that will be used in a heat sensitive recording apparatus that converts an electrical signal to a thermal signal and uses the Joule heat of a tiny thin film resistor thereby developing color in a heat sensitive recording medium to be recorded and also relates to the production of the heat.
The heat sensitive recording system that is a typical nonimpact system is widely and generally used since the heat sensitive recording system makes no noise and has other advantages. Since the glaze layer positioned under the heat generating section of a thermal head used in a heat sensitive recording system has great influence on the printing efficiency and printing quality, various studies have been made on the glaze layer.
Generally a glaze layer having a low heat conductivity is considered preferable to be used as a glaze layer of a thermal head, since such a glaze layer can secure efficiently the amount of heat required for printing by reducing the amount of heat escaping to the substrate of the head when electric power is applied through an electrode means to the heat generating section of a resistor. However, in practice, when the amount of heat for printing is to be sufficient for requirements, the thickness of the glaze layer must have at least a certain thickness or over, and if the thickness is too great, the accumulated amount of heat will become high. As a result, when an electric current is not applied, that is, when cooling is effected, the heat radiation will become relatively insufficient, and a quick lowering of temperature will not be attained, resulting in undesired printing. A glaze layer for a thermal head is thus required to have contradictory properties.
To deal with this, it has been proposed in Japanese Laid-Open Patent Application No. 74370/1983 that a glaze layer positioned under the heat generating section of a thermal head be made up of a glass simply having a great number of bubbles. Since this thermal head uses as a glaze layer a glass having a low heat conductivity and there are a great number of pores (bubbles) present in the glaze layer, the heat conductivity as a whole becomes low owing to the pores in comparison to one having no pores, and the heat accumulation becomes low at a certain temperature. That is, if a glaze layer having pores is formed to have the same appearance as a glaze layer having no pores, the glaze layer having pores has a lower heat conductivity and accumulates a lesser amount of heat than in the case of a glaze layer having no pores. Therefore the glaze layer made up of said material is excellent in that a thermal head comprising the glaze layer is improved in attaining of a printing temperature and a quick increase in temperature when electric power is applied, and has favorable heat response.
However, when such a glaze layer is to be produced in practice, many problems will be concurrently met. That is, generally when a glass having a great number of bubbles is to be produced, a paste of a glass powder having an average particle diameter on the order of about 10 to 50 .mu.m is calcined under certain conditions, during which bubbles are generated in the paste. When this is used for the production of a glaze layer of a thermal head, the calcining temperature must be controlled on the order of a preciseness of .+-.2.degree. to 3.degree. C. and this results in technically higher setting of the calcining conditions and technical difficulty. Further, if suitable calcining conditions are not maintained, the glass powder bubbles generated in the paste will not remain in the glaze layer but will appear at the surface and make the surface rough, which has an adverse effect on the printed quality in practice, and therefore there is a risk that a thermal head will be provided which is good in heat response but lacks commercial value.